3 Answers
3

How bigger the thing, how longer it will take to be fully baked. This is the reason why you find such a difference in the baking times for banana bread and banana muffins.

Surface Area

The volume can be the same (ground surface x height), but the surface exposed to the heat can differ. If something is flatter, the center is more rapidly reached. If you would bake your bread in a normal bread mold, it would take a longer time than if you would flatten out a bread on a baking sheet (but note that the volume is the same).

Oven temperature

Something very obvious. If it's hotter, it won't have to bake as long. However, don't play too much with this. I would advice to use the temperature a recipe has recommended. If you put it too hot, the outer parts can burn, when the inner parts would not be baked fully.
This question is related.

Vessel Material

Some materials conduct heat better than others. As a consequence, some things will be faster done in metal than in glass for example. (But I believe this has a less important role than the other factors I've mentioned. I can't say much about it, since I don't know a lot about it.)

Rather than volume and surface area separately, what often really matters is the maximum distance any of the interior is from the surface. Unless you're baking something really strangely shaped, you can probably think of this as thickness/depth.
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Jefromi♦Feb 16 '12 at 15:25

@Jefromi very good point :) However, I think it's more clear if I pull it apart. But if I'm mistaken, feel free to edit.
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MienFeb 17 '12 at 0:19

There are lots of variables, but in this case the answer seems obvious: volume of the product. It takes more time to heat all the big bread than a small muffin.

The other factors that could affect the time include:

the oven temperature used – the lower temperature, the more time is needed to pass the same amount of energy to the food

the process required to have the thing 'cooked' – protein denaturation (cooking an egg or tender meat) takes less time than starch gelatinization (baking bread or cooking potatoes) which takes less time than collagen hydrolysis (baking harder meat)

Like I said on Mien's answer, it's not exactly the volume; mostly it's the depth/thickness. Usually the two are correlated: the middle of a loaf of bread is farther from the surface than the middle of a muffin. But if for example you scaled something up from an 8x8 pan to a 9x13 pan, and kept the same depth, it probably wouldn't take longer to bake.
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Jefromi♦Feb 17 '12 at 1:18

@Jefromi I don't think you have the same depth if you bake something in a 9x13 vs in a 8x8 pan. Did you 13x5 or something? Or am I mistaken?
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MienFeb 18 '12 at 7:41

@Mien: Emphasis added: If you scaled something up... and kept the same depth...
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Jefromi♦Feb 18 '12 at 15:30

Because of the size. The bread is much bigger than a muffin, so it takes longer for the heat to reach the center of the bread.
Split the bread to muffin-size pieces and the cooking time will be the same.